Data source: SQLite plugin

Embedded file database. **SQL** — closest data model to the existing adapter, but same embedding constraint as DuckDB.

Builds on the **Seshat** foundation (#93, shipped in #99): host `tcp-client` (+ host-side TLS), owner-thread query execution, `secure-storage` for credentials, and the `UiNode` DSL (connection dialog, dock tabs, results grid, `json-tree`).

## Constraint (key risk)
- SQLite is a **library, not a server**. Under `wasm32-wasip1` we need either a wasm-compatible SQLite build linked into the plugin, or the **subprocess path** (`docs/DATABASE_PLUGINS.md` Option C). Most `rusqlite`-style crates need C and won't build to `wasip1` as-is.
- Opens a `.sqlite`/`.db` file via the existing `file-access` world.

## Reuse
- `DbAdapter`, results grid, schema browser.

## New work
- Choose + wire a wasm-friendly SQLite engine (or subprocess); file open via the file dialog.

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Part of the data-source plugins epic #109.